FIELD OF THE INVENTION
In optoelectronic device technology it is often necessary to align optical devices which are mounted on a planar substrate, such as a silicon chip, with other discrete devices, such as other optical devices mounted on the same chip, or a separate chip, or with optical fibers.
It is known to form alignment structures, such as pedestals and standoffs using conventional silicon waferboard technology. For instance, Reactive Ion Etching (RIE) may be used to etch polyimide standoffs and even silicon alignment pedestals may be formed by RIE. However, RIE technology is very difficult and expensive and raises environmental concerns due to the chlorine-based reaction technologies used therein.
Furthermore, due to the nature of RIE, it is extremely difficult, if not impossible, to form tall vertical alignment pedestals. Typically, the vertical relief of structures formed by RIE averages 10 microns. Additionally, the vertical profile of the walls of structures formed by RIE is not quite vertical, due to imperfect anisotropic etching, which results in the walls of the structures so formed being at angles less than 90 degrees from the horizontal.
The known method of forming vertical structures by RIE also suffers from the difficulty of forming alignment V-grooves in the silicon substrate after the formation of the vertical alignment structures. The previously formed vertical structures interfere with the application of the photoresist, generally applied by the spin-on technique, such that it is difficult to form a resist coating with a uniform thickness. Unfortunately, the V-grooves can not be formed prior to formation of the vertical alignment structures, since the thermal oxide RIE masking material, grown on the surface of the substrate, will also be grown on the surfaces of the V-grooves, thereby adversely affecting the accuracy of the V-groove alignment.
The use of polymeric alignment structures is less than desirable, especially in a high-throughput, production environment, due to processing difficulties, reliability concerns and poor accuracy in the z-axis (vertical).